Oxygen concentrators to produce breathable oxygen for a person requiring an oxygen-enriched atmosphere generally operate in the following manner. A compressor supplies compressed ambient air to a bed of molecular sieves. The molecular sieves adsorb nitrogen gas from the compressed ambient air to provide a gas with a high oxygen content. The oxygen-enriched gas then exits the bed of molecular sieves and passes through a regulator and a patient adjustable needle valve, which controls the gas flow rate. The oxygen-enriched gas can then be supplied to a patient who can breath the oxygen-enriched gas. In general, most oxygen concentrators contain two beds of molecular sieves. While one bed of molecular sieves is in operation to produce the oxygen-enriched gas, the second bed of molecular sieves is being purged of the adsorbed nitrogen in order to regenerate the bed of molecular sieves. The two beds of molecular sieves allow the oxygen concentrator to supply a continuous flow of an oxygen-enriched gas to the patient. Oxygen concentrators manufactured by Invacare®, Respironics®, and Sunrise® use two beds of molecular sieves for the creation of a continuous supply of an oxygen-enriched gas from a source of ambient air.
One of the problems that arises in the use of the molecular sieves is that the molecular sieves not only adsorb nitrogen, but also water vapor. Thus the oxygen-enriched gas being delivered to the patient can be extremely dry, typically with a dew point of −40° F. or lower (a relative humidity of less than 0.5%). The dry gas can cause dehydration of the nasal passages and respiratory system, which can lead to patient discomfort and irritation.
There are existing humidifiers for humidifying oxygen-enriched gas flowing to the patient. These humidifiers generally have a source of liquid water positioned to allow the oxygen-enriched gas to bubble through the liquid water, thus humidifying the oxygen-enriched gas. While these humidifiers work for humidifying the oxygen flow, they do have several major drawbacks. First, unless the water is re-supplied, eventually the water completely evaporates, ending all humidification. Second, standing water offers a site for bacterial growth. This is especially true since the water for the bubbler is usually, located on the exterior of the oxygen concentrator, and thus is open to environmental contamination.
In addition, bacteria growing in standing water can become aerosolized during the bubbling process and be carried along with the oxygen-enriched gas, potentially reaching to the patient. Third, manufacturers of oxygen concentrators often go to great lengths to minimize the noise output of their oxygen concentrators. Providing for a source of liquid water for humidifying oxygen-enriched gas located outside a cabinet of the oxygen concentrators and thus outside of the oxygen concentrators' noise abatement measures can contribute significantly to the noise generated by the oxygen concentrator through the noisy bubbling action.
The use of membrane devices to humidify oxygen-enriched gas is also known in the art. These membrane devices work by using selective membranes to transfer moisture from one gas to another gas without significant transfer of other components. This transfer of moisture from one gas to another gas is accomplished by using a membrane having a greater selectivity for water over the other components such as both oxygen and nitrogen. The selectivity of a membrane for water compared to oxygen and nitrogen is defined by the ratio of the water permeability to the permeability of either the oxygen or nitrogen. It is noted that the aforementioned selective membranes have a selectivity for water over oxygen or nitrogen of greater than 1, more preferably greater than 10, and most preferably greater than 100.
In use, the above-mentioned membrane device is in contact with both a high-pressure compressed stream of gas exiting the compressor and a lower-pressure oxygen-enriched stream of gas exiting a regulator and needle valve. Moisture passes from the high-pressure compressed stream of gas through the selective membrane to the lower-pressure oxygen-enriched stream of gas.
The use of membrane devices for gas humidification have advantages over oxygen concentrators that humidify their gases with bubblers. Firstly, the operator never needs to fill or refill the membrane devices with water as moisture for humidification is obtained from ambient air. Secondly, oxygen concentrators that humidify through the use of membrane devices are quieter than oxygen concentrators that humidify with bubblers as the membrane devices do not contribute to the sound produced by the oxygen concentrators.
Membrane devices such as the ones disclosed in the articles of Yonago Acta Medica, 1999; 42: 185-188 and Internal Medicine, Vol. 36, No. 12 (December 1997) do have one major problem in that membrane devices introduce the possibility of over humidifying the oxygen-enriched gas. This over humidification introduces the possibility of condensation and thus bacterial growth. More specifically, since membrane devices used in oxygen concentrators are usually installed down stream of the compressor, the partial pressure of the water vapor is frequently above the vapor pressure of water at room temperature. It is noted that since the stream of gas coming out of the compressor is usually at a temperature that is greater than the ambient temperature there is not necessarily condensation inside the membrane device. However, the lower-pressure stream of oxygen-enriched gas that enters the membrane device from the regulator and needle valve can become humidified to a partial pressure that is likely above the room temperature vapor pressure. This means that as the oxygen-enriched gas cools enroot to the patient, condensation can occur. This not only means that the patient can periodically receive liquid water, but also that there exists a risk of bacterial growth.
There are two current methods for dealing with the issue of over humidification by the membrane devices. Firstly, the membrane devices can be used in an environment where the ambient humidity never exceeds an amount that would cause the oxygen-enriched gas to become over humidified. However, since many of these devices are used in patient's home under a variety of environmental conditions, the ambient humidity is difficult to control. Secondly, a shunt can be installed so that a portion of the oxygen-enriched gas bypasses the membrane device, remaining at an extremely low humidity. When the streams of oxygen-enriched gas are later remixed, an optimal humidity can be achieved. This system however, requires adjustment by the user to match ambient conditions as well as requiring additional valves and tubing.